function download(res) {
    let blob = new Blob([res.data], { type: res.data.type })
    let downloadElement = document.createElement('a')
    let href = window.URL.createObjectURL(blob); //创建下载的链接
    downloadElement.href = href;
    downloadElement.download = res.headers['filename']; //下载后文件名
    document.body.appendChild(downloadElement);
    downloadElement.click(); //点击下载
    document.body.removeChild(downloadElement); //下载完成移除元素
    window.URL.revokeObjectURL(href); //释放blob对象
}